Note that the present disclosure is described in terms of a preferred embodiment of the invention for use with eddy current probes. However, the applicability of the invention is not limited to eddy current probes, and all applications of the invention to other types of non-destructive inspection probes, such as ultrasonic probes, are within the scope of the present disclosure.
Eddy Current (EC) is a commonly used method for non-destructive testing and inspection (NDT/NDI). Eddy Current Array (ECA) probes use multiple sensors in order to increase the inspection coverage. Using multiple excitation coils and receiving coils (sensors) allows the user to inspect a given surface faster than with a single EC sensor. Array probes using up to 32 sensors are not uncommon. However, due to constraints of size and cost, most ECA instruments have a limited number of electronic sensing inputs. For example, it is common for a compact hand-held ECA instrument to have as few as 4 electronic inputs for as many as 64 sensors. In such a case, an analog multiplexer is used to switch between multiple sensors at each electronic input.
FIG. 1A shows an example in existing practice of a multiplexer 4′ which has eight input channels from a sensor array 2′, allowing data from up to eight sensors to be acquired with a single electronic sensing module 10′. Sensing module 10′ comprises an amplifier 6′ and a digitizer 8′.
A common method of achieving multiplexing is known in the art as Time Division Multiplexing (TDM). TDM is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches which cause each signal to appear on the line for only a fraction of time in an alternating pattern. An ECA instrument would thus sample every sensor in such a way that the input signals from each sensor are periodically sampled, with each sensor being allocated a time slot equal to the period of the signal. FIG. 2A shows an example of a sinusoidal excitation signal, scanning through four input sensors with four sequential time slots each having a duration equal to the period of the excitation signal. It is understood that the signal pattern shown in FIG. 2A is repeated, thereby repeatedly cycling through the input from the four sensors.
TDM is commonly used in existing practice, but, as will be described below, the method imposes a severe limitation on the maximum inspection speed, particularly for low frequency ECA probes.
Therefore there exists a need in existing practice for a method of multiplexing which allows improved productivity by achieving a higher inspection speed.